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High Transition Temperature Ceramic SUPERCONDUCTORS

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  1. High Transition Temperature Ceramic SUPERCONDUCTORS 5월 1일 이종규

  2. Resistance/Ω 0 4.0 4.1 4.2 4.4 4.3 Temperature/K 4.7.1 Background • The history of superconductivity has its origins in the experiments of Kamerlingh Onnes who in 1908 at the University of Leidan succeeded in liquefying helium and reaching temperature down to 1K • In 1911, he discovered that as the temperature was reduced the resistivity fell sharply at about 4K and at the transition temperature Tc fell to zero (in mercury) • Principle among them being tin(3.7K), tantalum(4.5K), lead(7.2K), niobium(9.2K), Nb3Sn(18K) and Nb3Al0.5Ge0.2(20.9K)

  3. 4.7.1 Background • Meissner effect ( by W. Meissner & R. Oschenfeld in 1911 ) - when a magnetic field is applied to a superconductor at a temperature below Tc surface currents flow so that the magnetic field they generate just cancels the applied field within the material. • Because the net flux within the material is zero the superconductor behave like a perfect diamagnet. • ‘Screen current’ induced by an applied magnetic field, those that are generated by an applied emf are also confined to the surface of the superconducting material.

  4. 30 20 Hc/MAm-1 10 20 10 T/K 10 log( jc / Am-2 ) 8 6 20 10 H/MAm-1 4.7.1 Background • 초전도체는 임계전류밀도(Jc), 임계자장(Hc), 임계온도(Tc) 이 세가지중 어느 한가지라도 벗어나는 순간 상전도체로 상전이 • In summary, Jc depends on the applied magnetic field and Hc on the conduction current density, and both are quite strongly dependent on temperature below Tc. Nb3Sn

  5. 4.7.1 Background • BCS theory ⇒ J. Bardeen, L.N. Cooper, J.R. Schrieffer가 초전도 현상을 설명하기 위해 도입한 물리학 이론. ( in 1957 ) • Cooper pairs ( by L. N. Cooper) • 전자와 격자의 상호작용에 의해 전자간에 인력이 생겨 전자쌍이 형성되고, 초전도 상태에서 격자진동(Lattice vibration)의 도움을 받아 에너지를 잃지 않고 저항 없이 이동. • 일정온도 이상으로 올라가면 전자들의 에너지가 높아서 쿠퍼쌍은 깨지게 되고, 상전도체로 전이.

  6. 4.7.1 Background

  7. 4.7.1 Background • Discovery of superconductivity in the La-Ba-Cu-O system. ( in 1986 ) ▫ Particular compound observed – (La,Ba)2CuO4 – has the layer-perovskite K2NiF4 structure. ▫ Replacement of lanthanum by barium affects the Cu3+/Cu2+ ratio and gives a broad transition to the superconducting state around 35K. • Discovery of superconductivity in the YBa2Cu3O7-d at 93K.( in 1986 ) ▫ so called 123 material. ▫ The structure is closely related to that of perovskite. ▫ At the stoichiometric composition YBa2Cu3O7, both Cu2+ and Cu3+ exist, and Cu+ can also be present at stoichiometric YBa2Cu3O6. ▫ In comparing with that for three cells of the perfect perovskite Ca3Ti3O9, it’s clear that the 123 compositions contain vacant oxygen site. Therefore, allowing for the existence of Cu2+ & Cu3+, the composition can be written

  8. Superconductivity in (La,Ba)2CuO4 The Structure of YBa2Cu3O7-d 4.7.1 Background

  9. 4.7.1 Background • The perovskite structure seems to favour the existence of soft modes (low - frequency phonons) as evidenced by its tendency to structural instability, e.g. ferroelectric-paraelectric transition. • 123 compound exhibits a tetragonal-orthorhombic transition in the region of 700°C (the exact temperature depends on the value of d) • Because of the Jahn-Teller effect the Cu-O distances in the structure depend on the oxidation state of the Cu ion so that on transfer of an electron from, say, Cu2+ to Cu3+ there will be an adjustment in the lattice parameters.

  10. 4.7.2 Processing • Ceramic superconductors can be fabricated using standard processing routes. • In order to maintain control over the oxygen constant (d value) special attention has to be given to high-temperature annealing procedures and to the oxygen content of the annealing atmosphere. • Thin film can be fabricated by sputtering and these have potential for the manufacture of electronic devices. • Concerted efforts are being made o fabricate superconducting ‘wire’ in which the ceramic is contained in a metal tube. • To avoid the effects of grain boundaries and other complexities inevitable in ceramics, attempts are being made to grow single-crystal material.

  11. 4.7.3 Applications • The following extract from a 1968 publication makes interesting reading in the light for recent discoveries. • 물리적 현상에서의 산업적 관심사는 그 현상의 ‘품질’ 혹은 ‘외향적 형태’이다. 사실 전기저항이 없다는 것은 새로운 가능성이 풍부하다는 것을 의미한다. 지금까지 발견된 가장 높은 전이온도가 20.05 °K일지라도, 초전도체가 어떤 장점이 있는 지 알기 위해 런던 같은 인구가 밀집한 지역에서의 전력공급용 케이블에 시험적으로 사용되고 있다. 또한 ‘cryotrons’같은 논리회로 소자용 초전도체에도 관심이 많다. 초전도체의 매우 흥미 있는 가능성 중에는 높은 자장을 발생시키는 초전도 자석도 포함되는데, 기술적 응용이 충분히 가능할 만큼 10 Wb/m2(1000000G) 이상의 자속 밀도를 발생시킬 수 도 있다. 한가지 기대되는 것은 전이온도가 50 ~ 100 °K정도의 초전도물질이 만들어진다면 과연 전기기술에서 어떤 획기적인 변화가 발생 하는가이다. 앞으로 20 °K이상의 전이온도를 갖는 물질을 찾지 못하더라도 초 전도체의 기술적 응용은 계속 되리라 예상된다.

  12. 4.7.3 Applications • Since then there has been the unexpected breakthrough with the superconducting state being achieved at liquid nitrogen temperature. • 전력공급용 cable ⇒ 현실적으로 처리해야 할 문제점이 많다. • Interconnections in computer ⇒ 용량성 소자의 기능을 향상시켜 신호 처리속도 증가 • Antenna ⇒ 손실이 없다는 특성 때문에 안테나 디자인에 유리하다. Meissner effect때문에 자장간섭에 영향을 받지 않는다. • Fusion ⇒ 핵융합 플라즈마에서는 높은 자장의 발생이 핵심. • NMR(핵자기공명장치), MRI(자기공명영상장치) ⇒ 의학 진단 • Magnetically levitated transport systems(자기 부상 수송) ⇒ 자기 부상 열차(초전도체의 반자성의 특성 이용)

  13. 4.7.3 Applications • Josephson effect ⇒ Quasi-particle tunneling : 두 초전도체를 두께가 대략 수십Å정도인 부도체 막 사이로 접근시키면 전자의 파동함수가 겹쳐 양자역학적 터널링이 일어남. ( in 1960 by I. Giaever) ⇒ 1962년 B.D. Josephson이 예언하고, P.W 앤더슨 등이 실험적으로 확인. ⇒ Cooper pairs should be able to ‘tunnel’ through a thin(≈10-9 m) barrier from one superconductor to another with no electrical resistance. ⇒ I0 depends on the overlap of the wavefunctions across the barrier and, in, turn, on the barrier width. ⇒ d0 is a measure of the difference in phase at zero time of the wavefunctions of each side of the barrier.

  14. 4.7.3 Applications A Josephson junction Non-linear voltage – current characteristic of a Josephson junction

  15. 4.7.3 Applications • DC Josephson effect ⇒ Josephson junction을 통해서 직류전류를 흘려 보낼 때, 임계전류까지는 절연막이 있음에도 불구하고 초전도체 사이에 전위차가 발생하지 않고 직류전류가 흐른다. • AC Josephson effect ⇒ 직류전류가 임계전류를 넘어서면 두 초전도체 사이에 전위차가 발생하여 그 때부터는 교류전류가 흐르게 된다. • 이 때 교류주파수는 직류전압에 비례하고, 외부로부터 m-wave를 조사하면 간섭현상이 생겨, 교류주파수가 전자기파 주파수와 같을 때 직류전류가 발생한다. • The Josephson junction can be switched between the superconducting and resistive states in very short times of the order of 10-12 s and there is the possibility of its exploitation in fast switching logic circuits.

  16. 4.7.3 Applications • SQUID(Superconducting Quantum Interference Devices) ⇒ If the junction is irradiated with microwaves of frequency f, the current-voltage characteristic has a series of steps as shown below • Because the frequency can be measured very accurately(to approximately 1 in 1013) in terms of the caesium atomic clock, the voltage steps are determined to a high accuracy(approximately 1 in 108) → Basis for defining voltage standards